1. Field of the Invention
The present invention relates to inspection systems and methods, and more particularly to a method and an apparatus for determining integrity of critical bonded or bolted joints in aerospace hardware assemblies.
2. Background of the Invention
Considerations for developing viable aerospace transportation systems of the future will include high safety and reliability, as well as low cost and maintenance. Advanced composite structural components are likely to be a key element in realizing the greatest benefits. This is because composites offer the greatest flexibility and versatility in carrying sensors for system health monitoring and repair diagnosis.
Composite components typically incorporate bonded or bolted joints. Such joints are particularly efficient in transferring load between structural components, but in bonded joints adhesive internal shear stresses cannot be monitored. Therefore, the strength of the bonded joint, which may include unseen disbonds, is not known until failure occurs. With mechanically fastened joints, however, the failures occur as a result of fatigue, i.e., stress concentrations around the fastener hole.
Various types of sensor equipment have been contemplated for monitoring the integrity of bonded joints to detect failures, i.e., disbonds and delaminations, as well as for assessing the strength of attachment of the members. One such sensor is the strain gage, a device which is expensive to install, unreliable in harsh environments, and provides measurements only at a point.
More recently, fiber optic strain sensors have been recognized as a very attractive mechanism for performing these tasks, which include the measurement of strain, vibration and other phenomena, e.g., cracks, fatigue and delaminations. These sensors typically include fiber optic cables embedded within a laminated composite, and an optics/electronics system for generating signals used in the sensing process.
Fiber optic strain sensors possess several inherent advantages over existing inspection systems. In contrast to strain gages, which provide measurements only at a point, a fiber optic strain sensor returns an integrated strain along the fiber length. When coupled with available electronics, a single fiber can monitor, in flight, a large bonded area with unparalleled accuracy. However, successful implementation of fiber optic sensors in aerospace vehicle structures has, until now, been hampered by design, manufacturing and operability constraints.
There is, therefore, a great need for a fiber optic strain sensor system which is easily outfitted in aerospace vehicle structures and which returns an integrated strain along the fiber length. Moreover, if real-time distributed inspection of critical composite joints in aerospace structures could be achieved using networked fiber optic sensors, a significant contribution would be made in assuring the reliability and safety of this country's current and future generation of aerospace vehicles.